Add and use stability helper methods

This avoids an ambiguity (when reading) where `.level.is_stable()` is
not immediately clear whether it is general stability or const
stability.

compiler/rustc_middle/src/ty/context.rs

@@ -2791,7 +2791,7 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn is_const_fn(self, def_id: DefId) -> bool {
         if self.is_const_fn_raw(def_id) {
             match self.lookup_const_stability(def_id) {
-                Some(stability) if stability.level.is_unstable() => {
+                Some(stability) if stability.is_const_unstable() => {
                     // has a `rustc_const_unstable` attribute, check whether the user enabled the
                     // corresponding feature gate.
                     self.features()